Golf balls, whether of solid or wound construction, generally include a core and a cover. It is known in the art to modify the properties of a conventional solid ball by altering the typical single layer core and single cover layer construction to provide a ball having at least one mantle layer disposed between the cover and the core. The core may be solid or liquid-filled, and may be formed of a single layer or one or more layers. Covers, in addition to cores, may also be formed of one or more layers. These multi-layer cores and covers are sometimes known as “dual core” and “dual cover” golf balls, respectively. Additionally, many golf balls contain one or more intermediate layers that can be of solid construction or, in many cases, be formed of a tensioned elastomeric winding, which are referred to as wound balls. The difference in play characteristics resulting from these different types of constructions can be quite significant. The playing characteristics of multi-layer balls, such as “feel” and compression, can be tailored by varying the properties of one or more of these intermediate and/or cover layers.
Manufacturers generally provide the golf ball with a durable cover material, such as an ionomer resin, or a softer cover material, such as balata or polyurethane. Balata is a natural or synthetic trans-polyisoprene rubber that is a favored cover material for highly-skilled golfers because of its softness. Balata-covered balls, however, are easily damaged, and therefore lack the durability required by the recreational golfer.
Ionomer resins have in many situations replaced balata as a cover material. Chemically, ionomer resins are a copolymer of an olefin and an α,β-ethylenically-unsaturated carboxylic acid having 10-90% of the carboxylic acid groups neutralized by a metal ion and are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization. Commercially available ionomer resins include copolymers of ethylene and methacrylic or acrylic acid neutralized with metal salts. Examples include SURLYN® from E.I. DuPont de Nemours and Co. of Wilmington, Del. and IOTEK® from Exxon Corporation of Houston, Tex.
Surrounding the core with an ionomeric cover material provides a ball that is virtually indestructible by golfers. The core/cover combination permits golfers to impart a high initial velocity to the ball that results in improved distance. Because the materials used to form solid golf balls are very rigid, however, solid balls generally have a hard “feel” when struck with a club.
A number of patents have been issued that are directed towards modifying the properties of layers used in forming wound balls, conventional solid balls, multi-layer balls having dual cover layers, dual core layers, and/or balls having a mantle layer disposed between the cover and the core. For example, U.S. Pat. No. 3,147,324 is directed to a method of making a golf ball having a polyurethane cover.
Polyurethanes are used in a wide variety of applications including adhesives, sealants, coatings, fibers, injection molding components, thermoplastic parts, elastomers, and both rigid and flexible foams. Polyurethane can be produced by the product of a reaction between a polyurethane prepolymer and a curing agent. The polyurethane prepolymer is generally a product formed by a reaction between a polyol and a diisocyanate. The curing agents used previously are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Since about 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a fast-reacting diamine. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol.
Additionally, U.S. Pat. No. 3,989,568 discloses a three-component system employing either one or two polyurethane prepolymers and one or two polyol or fast-reacting diamine curing agents. The reactants chosen for the system must have different rates of reactions within two or more competing reactions.
U.S. Pat. No. 5,692,974 discloses methods of using cationic ionomers in golf ball cover compositions. Additionally, the patent relates to golf balls having covers and/or cores incorporating urethane ionomers. Improved resiliency and initial velocity are achieved by the addition of an alkylating agent such as t-butyl-chloride, which induces ionic interactions in the polyurethane to produce cationic type ionomers.
International Patent Application WO 98/37929 discloses a composition for golf ball covers that comprises a blend of a diisocyanate/polyol prepolymer and a curing agent including a blend of a slow-reacting diamine and a fast-reacting diamine. Improved “feel,” playability, and durability characteristics are exhibited.
One problem manufacturers have encountered with many polyurethane compositions is the tendency of these elastomers to react with molecular oxygen in a degradation process called “autoxidation.” This degradation process results in undesirable changes, such as product discoloration and loss of physical properties. Autoxidation may be initiated by heat (thermo-oxidative degradation), high energy radiation (photodegradation), mechanical stress, catalyst residues, or through reaction with other impurities. However, photodegradation by ultraviolet (“UV”) radiation is believed to be the most damaging of these autoxidation mechanisms. Thermo-oxidation and photodegradation processes are initiated with the formation of free radicals. These free radicals react rapidly with oxygen to from peroxy radicals. These peroxy radicals may further react with the polymer chains leading to the formation of hydroperoxides. On exposure to additional heat or light, hydroperoxides decompose to yield more radicals that can reinitiate the degradation process.
UV absorbers protect against photodegradation by “competing” with the polymer for absorption of ultraviolet light. An ideal UV absorber should be very light stable and should have broad, intense absorption over the UV range from about 290 nm to 400 nm.
Antioxidants, on the other hand, interrupt the degradation process in different ways according to their structure. The major classifications of antioxidants are primary antioxidants and secondary antioxidants. Primary antioxidants, such as sterically hindered phenols, react rapidly with peroxy radicals (ROO.) to break the degradation cycle. Secondary antioxidants, such as arylamines, are more reactive toward oxygen-centered radicals than are hindered phenols. The secondary antioxidants react with hydroperoxide (ROOH) to yield non-radical, non-reactive products, and are frequently called hydroperoxide decomposers.
The color instability caused by both thermo-oxidative degradation and photodegradation typically results in a “yellowing” or “browning” of the polyurethane layer, an undesirable characteristic for urethane compositions are to be used in the covers of golf balls, which are generally white.
Manufacturers have found that the use of primary antioxidants helps suppress the formation of free radical species and hydroperoxides, and that the use of UV absorbers and hindered amine light stabilizers (“HALS”) helps protect polyurethanes from UV light-induced oxidation, either or both of which can result in unwanted discoloration of polyurethane-formed components. Antioxidants include primary antioxidants, such as sterically hindered phenols, that can react rapidly with peroxy radicals, secondary antioxidants, such as hydroperoxide decomposers, that react with hydroperoxide to yield non-radical, non-reactive products. Multifunctional antioxidants, such as hindered amine stabilizers (“HAS”), combine the functions of primary and secondary antioxidants and are known to those of ordinary skill in the art to be structurally interchangeable with HALS. Various references describing such compositions are set forth below.
U.S. Pat. No. 5,156,405 to Kitaoh et al. discloses a golf ball having a core, an ionomer cover, and a clear paint outer layer, where the cover and the outer layer each comprise a light stabilizer and a UV absorber. The light stabilizer and a UV absorber are present in the cover in an amount of from 0.1 to 5 parts, based on 100 parts by weight ionomer resin, and in the clear paint in an amount of from about 0.1 to 20 parts, based on 100 parts by weight paint solid. The ratio of the amount of light stabilizer ranges from 75/25 to 25/75 in the cover and from 75/25 to 5/95 in the clear paint.
U.S. Pat. No. 5,494,291 to Kennedy discloses a golf ball having a fluorescent cover and a UV light blocking, visibly transparent topcoat. The cover contains a fluorescent material that absorbs at least some UV light at wavelengths greater than 320 nm and emits visible light.
U.S. Pat. No. 5,692,974 to Wu et al. discloses golf balls which have covers and cores and which incorporate urethane ionomers. The polyurethane golf ball cover has improved resiliency and initial velocity through the addition of an alkylating agent such as t-butyl chloride to induce ionic interactions in the polyurethane and thereby produce cationic type ionomers. UV stabilizers, antioxidants, and light stabilizers may be added to the cover composition.
U.S. Pat. No. 5,823,890 to Maruko et al., discloses a golf ball formed of a cover of an inner and outer cover layer compression molded over a core. The inner and outer cover layers should have a color difference ΔE in Lab color space of up to 3.
U.S. Pat. No. 5,840,788 to Lutz et al. discloses a UV light resistant, visibly transparent, urethane golf ball topcoat composition for use with UV curable inks. The topcoat includes an optical brightener that absorbs at least some UV light at wavelengths greater than about 350 nm, and emits visible light, and a stabilizer package. The light stabilizer package includes at least one UV light absorber and, optionally, at least one light stabilizer, such as a HALS.
To date, it has been difficult for manufacturers to properly attain the desired long-term appearance of polyurethane compositions used in golf ball covers without adversely affecting golf ball performance. Many golf balls have at least one layer of “paint” covering the cover material, which paint can be a clear and/or pigmented topcoat material. Those of ordinary skill in the art should readily recognize that repeated blows with a golf club tend to result in exposure of the cover material to harmful UV rays for hours at a time, resulting in undesirable “yellowing” and/or “browning” of the cover material. Further, absent exposure of the polyurethane composition to direct UV radiation, degradation still occurs over a long time period and the resultant discoloration can “bleed” through the paint layer, also discoloring the golf ball cover. This long-felt problem in the golf ball art has now led the Applicants to seek a desirable formulation of a polyurethane composition suitable for use in golf ball covers that exhibits improved color stability upon prolonged exposure to UV light.